1. Field of the Invention
The present invention relates to a cyclic training sequence group for uplink/downlink synchronization in the multiuser system, e.g., a Round-Robin training sequence group, and especially relates to a communication system and communication method using the cyclic training sequence group.
2. Description of the Related Art
Currently, there are many classical synchronization algorithms for performing downlink synchronization [1]-[7]. However, uplink synchronization is generally more difficult in the multiuser system, especially when a plurality of users adopt full-multiplexing transmission mode, because in uplink transmission, different users are different distances from the base station, which will introduce different timing offsets with respect to the base station. In addition, different users use different oscillators and the central frequencies of these oscillators may be different; and the moving speeds of different users are different so that different Doppler frequency offsets are formed. The above two reasons lead to users having different carrier frequency offsets with respect to the same base station.
Many classical algorithms utilize the repeated information in the training sequence to perform timing synchronization and carrier frequency offset estimation. At the base station, there will be a phase angle deviation between the repeated information in the training sequence due to the influence of the carrier frequency offset, which can be used to estimate the carrier frequency offset of the user. In the multiuser system adopting full-multiplexing transmission mode, when the training sequences transmitted by different users overlap at the base station, if these training sequences are identical or partially identical in structure, the corresponding phase angle deviation will contain a plurality of users' carrier frequency offset information, i.e., the carrier frequency offset information of a single user will be lost. The interference between training sequences may lead to the failure of the synchronization algorithm of some users.